1. Field of the Invention
This invention relates to a structure of a cylinder head of an engine, and more particularly to a structure of a cylinder head of a multiple-cylinder, in-line engine.
2. Description of the Prior Art
Generally, the cylinder head of a multiple-cylinder, in-line engine is fixed to the cylinder block by head bolts inserted into bolt holes which are formed in the cylinder head at its four corners and at its side walls at positions opposed to portions of the cylinder block between adjacent bores. The clamping force of the head bolts, combustion pressure acting on the cylinder head and thermal strain due to combustion head are apt to cause problems. For example, in the arrangement shown in FIG. 1 where the cylinder head B is fixed to the upper surface of the cylinder block A with a gasket E sandwiched therebetween by head bolts D inserted into bolt holes C formed in the side walls of the cylinder head B at positions opposed to the portions of the cylinder block A between adjacent bores, the part of the lower deck F of the cylinder head B between the bolt holes C tends to be bowed upwardly as shown by the chained line in FIG. 1, whereby the contact pressure between the lower deck F and the upper surface of the cylinder block A is lowered at the center of the lower deck F. This tendency is especially significant when the cylinder head B is made of an aluminum alloy. Such upward bowing degrades the seal between adjacent cylinders. When the space between adjacent bores is reduced in order to miniaturize the engine, the seal between adjacent cylinders is further deteriorated to cause so-called gas-escape between the adjacent cylinders. Further, in a so-called same-flow type engine where the intake ports and the exhaust ports open on the same side of the cylinder head, rigidity on the opposite side of the cylinder head is lowered, and accordingly the problem of seal deterioration due to the deformation of the cylinder head is more significant.
In Japanese Unexamined Utility Model Publication No. 53(1978)-123307 is disclosed a structure of a cylinder head in which, as shown in FIG. 2, the cylinder head 1 is made of an aluminum alloy and a thermal insulating member 5 having a thermal conductivity lower than that of the aluminum alloy is embedded in a partition wall 3 between adjacent combustion chambers 2 along the surface of a gasket 4. The thermal insulating member 5 can act also as a reinforcement which increases the rigidity of the partition wall 3, whereby the problem described above can be avoided. However, this approach is disadvantageous in that the step of embedding the thermal insulating member 5 into the partition wall 3 during casting of the cylinder head is very troublesome and adds to the manufacturing cost.